Tests performed before the deadly blowout of BP's oil well in the Gulf of Mexico should have raised doubts about the cement used to seal the well, but the company and its cementing contractor used it anyway, investigators with the president's oil spill commission said Thursday.

It's the first finding from the commission looking into the causes of the April 20 explosion that killed 11 workers and led to the largest offshore oil spill in U.S. history. And it appears to conflict with statements made by Halliburton Co., which has said its tests showed the cement mix was stable.

The company, which has blamed BP's well design and operational decisions for the disaster, acknowledged in a six-page statement released late Thursday that it never tested the final mixture of cement for stability after BP made a last-minute change to the mix...